Polymer latices comprising particles of micron size are typically prepared by suspension polymerization, in which an oil-soluble polymerization initiator is dissolved in a vinyl monomer and the mixture is poured into water in the presence of a protective colloid to effect polymerization in a suspended state. This technique, however, encounters with great difficulty in control of dispersed particles so as to have a narrow size distribution.
In order to overcome this disadvantage, it has been proposed to obtain a monodisperse polymer by seed polymerization as described in Japanese Patent Application (OPI) No. 106554/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). According to this process, fine polymer particles having a size of about 0.5 .mu.m are once produced by emulsion polymerization, which are then allowed to grow through further polymerization
On the other hand, it is known that monodisperse polymer particles of micron size can be obtained through one-step polymerization by solution polymerization in a heterogeneous system (hereinafter referred to as "solution system dispersion polymerization"), as taught in J. Polym. Sci. Polym. Lett. (ed.), Vol. 23, 103-108 (1985), in which a monomer is polymerized as dissolved in a solution capable of dissolving the monomer but incapable of dissolving a produced polymer.
The aforesaid seed polymerization involves a larger number of stages for growth of polymer particles according as the desired particle size becomes greater, which is disadvantageous from economical standpoints. The solution system dispersion polymerization has an advantage in that fine polymer particles having a uniform size of about 10 .mu.m at the greatest can be obtained, but the particle size as well as its distribution varies to a great extent depending on the polymerization conditions, such as a temperature and an amount of a polymerization initiator used.